This invention relates generally to a high resolution, image projection system employing at least one linear array of laser sources, suitable for displaying raster scanned monochrome, two color and full color images such as television video or computer generated text and graphics by projection onto a screen.
Conventional cathode ray tube (CRT) display devices become impractical for large screen sizes. The largest CRT consumer TV display generally does not exceed 40 inches measured diagonally across the screen.
For graphics image display, the ability of the CRT within a monitor to produce a proportional number of pixels for a unit screen area decreases as the tube size gets larger. This limits the ability of larger CRTs to produce the fine detail needed in graphics displays viewed at close distances. This occurs because of the difficulty in accurately reproducing the super fine red, green, and blue (RGB) phosphor trios or stripes over a larger face area without error during the manufacturing process. Furthermore, errors introduced in the deflection of the electron beam are magnified by the distances associated with the larger tube and cause spatial distortion of the pixel information on the screen.
In contrast to graphics displays, consumer TV displays must be bright and have high contrast. Since consumer TV displays are viewed at much greater distances than graphics displays, resolution is not an overriding factor. Additionally, CRTs generate low frequency electromagnetic fields and X-rays. Methods that are used to achieve high brightness cause the electron beam spot size to increase dramatically. As a consequence, this larger spot size causes a large reduction in image sharpness, that in turn reduces contrast and results in soft looking images. Additionally, larger TV screens require a larger deflection angle to prevent the CRT from becoming excessively long. Large deflection angles (over 90 degrees), coupled with large screen size, cause noticeable errors in linearity and color purity. Modern television CRTs were never intended to be large. The size, weight, and power trade-offs do not scale economically for large CRT systems.
As a result, video projectors using superposition of three CRT light sources for the primary colors of white light, namely red, green, blue (RGB), are commonly used. A standard product configuration uses three small diameter primary color CRTs and lenses to converge three separate colors at a screen image. Larger projection screen sizes, than those obtainable using a CRT, can be obtained using this method; however, the brightness and contrast are poor compared to that of a CRT used for home TV video viewing.
Another approach uses a single incandescent white light source to generate the primary colors that illuminate a LCD panel(s). The RGB pixels are independently modulated by the liquid crystal display ("LCD") selection matrices, that also generate the rastering. Although these projectors have fair resolution, there are other unavoidable problems related to this scheme. The incandescent white light source has a relatively short operating life and generates relatively large amounts of heat. The LCD devices cannot be manufactured without some minimum number of defects that, in turn, manifest themselves as permanent image artifacts on the screen regardless of the graphic or video source. Using LCD devices to generate the raster introduces a fixed and permanent resolution to the display device, making it very difficult to adapt the electronics to accept other resolutions for display of graphics and text information.
Brighter video projectors have been constructed using lasers. Typically, the green and blue beams are generated by argon ion gas lasers that directly emit green and blue radiation; the red beam is usually generated by a liquid dye laser (pumped with part of the high power blue and green gas lasers). Generally, each of the three light beams is independently modulated to produce the same luminance and chrominance represented by an input video signal. The three modulated beams are then combined spatially by optical means to produce a single so-called "white light" beam and directed toward the viewing screen by an appropriate raster/scanner optical system. Since only a single white light beam is projected toward the viewing screen in present gas-laser-based projection systems, they are of the N=1 type, the number N specifying the number of white light beams. In such systems, generally, a full color picture (or frame) is produced at the viewing screen by projecting a series of pixels using a combination of rotating and deflection mirrors. With proper synchronization, the rotating mirror scans the white light beam horizontally across the screen, sequentially painting a row of pixels; the deflection mirror simultaneously moves the white light beam vertically down the screen, filling out the picture frame one line of pixels at a time. At any given instant, the white light beam illuminates a given pixel in the frame with the appropriate luminance and chrominance.
Present laser-based N=1 video projector systems are generally capable of producing a brighter image than non-laser based systems, and they can achieve close to 100% color saturation. They also exhibit pixel size stability, since the pixel size is independent of white light beam power. In order to produce a reasonably bright image on a screen larger than 40 inches, the white light beam power at the screen should be in excess of three watts. Gas lasers used in present N=1 projection systems have power efficiencies typically &lt;0.1 percent; accordingly, present day gas-laser-based projection systems require several kilowatts of conditioned electrical power and conditioned cooling water sufficient to remove several kilowatts of waste heat. Such systems are therefore relatively big, lack easy portability and are expensive. In present N=1 laser-based systems, three separate acousto-optic (AO) light modulators are used to impress video modulation information on each of the three RGB beams that forms the single white light output beam. These modulators are problematic and costly. At the power levels needed for bright large-screen displays, modulation nonlinearities and other undesirable effects can degrade picture quality.
Associated with conventional laser-based video projectors has been the need to use high speed components, both mechanical for scanning and electronic for modulation in order to produce a standard television picture. Current NTSC television pictures are reproduced at the rate of 1/30th of a second per frame, with each frame being filled by 525 horizontal scan lines. In a laser video projector, a multi-facet right polygon mirror is typically used for scanning the single white light laser beam across a screen. Even a 48-facet mirror would require an angular velocity in the region of 50,000 rpm. Bearings capable of such performance are very expensive. The scanning problem becomes even more critical when dealing with High Definition Television (HDTV) or high resolution graphics, since the pixel density increases, thereby requiring an even higher angular velocity of the polygon mirror.
Various video standards, such as NTSC, and variations of HDTV and computer standards already exist, and new standards will in time be proposed. Among other considerations, these video standards may differ in resolution, picture aspect ratio, frame rate, and interlacing method. Therefore, it would be desirable to have an image projector capable of, or easily adapted to, displaying video pictures conforming to various existing or future video standards.
Thus, there is a tremendous need for a bright, low cost compact video and graphic image projector capable of displaying multiple resolutions of video pictures such as NTSC, HDTV, and high resolution graphics pictures. Furthermore, since the requirements for display of graphics and TV video are different, it would be very desirable to combine the display function of both types of images into a single portable projection unit.